The invention relates to a system for predicting desynchronization of the synchronous machine, wherein desynchronization is predicted by detecting a dangerous state of the synchronous machine which is apt to move into an asynchronous state.
When the synchronous machine connected to an electric power system falls into an asynchronous (step out) state, the whole system is liable to fall into an asynchronous state. Accordingly, when the synchronous machine is subjected to a dangerous desynchronization, some convenient arrangements such as a field control or a separation of the synchronous machine from the system are required to prevent the system from falling into an asynchronous state.
Hitherto, there is known a desynchronization detecting system for the synchronous machine which comprises a combination of power relays and impedance relays as shown in FIG. 1 which is so arranged that when a resistance component R is represented by the abscissas and a reactance component X is represented by the ordinates, a power relay W.sub.1 operates in the region as hatched in FIG. 1 with a boundary of the straight line which is parallel with an axis of the abscissas and passes on the predetermined point "a" on a R-axis, a power relay W.sub.2 operates in the region as hatched in FIG. 1 with the boundary of the straight line which is parallel with the axis of abscissas and passes on the predetermined point "-a" on the R-axis and an impedance relay Z operates in the region interior of a circle with a radius of a predetermined impedance Z (=.sqroot.r.sup.2 +x.sup.2, wherein r is a resistance and x is a reactance). When the synchronous machine moves into an asynchronous state so that an equivalent impedance viewed from an installation point of the relays shifts tracing the locus of a curve "Z" from the point Z.sub.O of the normal operation, the related operations of the power relays W.sub.1, W.sub.2 and the impedance relay Z commence to operate a desynchronization detecting sequency circuit (not shown) for detection of the desynchronous state.
The conventional system, however, detects the desynchronization only after the desynchronization of the synchronous machine and has no function to predict desynchronization when the machine resides in the dangerous state of desynchronization.
Another conventional system with a function of predicting desynchronization is a desynchronization predicting system comprising stability limit control relays. This conventional system when applied, for example, to a synchronous generator includes a stability limit control relay "G" which actuates in the hatched region with the boundary of the curve "g" as shown in FIG. 2 wherein an axis of abscissas represents an active power P, an axis of ordinates represents a reactive power Q, a curve L represents an output limit curve of the synchronous generator and a curve "m" represents a steady-state stability limit curve and when the vector coordinates W(=P+jQ) of an output of the synchronous generator shifts tracing a locus of the curve "s" from the normal operation point W.sub.O over the stability limit curve "g" into the asynchronous region 1. The stability limit control relay G works to increase an excitation of the synchronous generator so as to withdraw the output vector coordinates W to the normal position for prevention of the machine from shifting into the asynchronous state.
However, either the desynchronization predicting system with the stability limit control relay or the desynchronization detecting system with the power relays and the impedance relays in combination utilizes as an input for detection, the external electric values such as a voltage across terminals, an output current, an impedance, an output, a reactive power, a phase angle and the like of the synchronous machine but is not responsive to an abnormal condition of the physical values in the synchronous machine so that it is doubtful whether the detected results represent the real position of the synchronous machine.